This invention relates to semiconductor devices, for example rectifier diodes, field-effect transistors, bipolar transistors and thyristors, each having a p-n junction which is operated under reverse bias in at least one mode of operation of the device.
U.S. Pat. No. 4009483 describes a semiconductor device comprising a semiconductor body having a body portion of one conductivity type adjacent a major surface of the body, and an active device region of the oppposite conductivity type also adjacent said major surface and forming with said body portion a main p-n junction which is operated under reverse bias in at least one mode of operation of the device. At least one annular region of said opposite conductivity type is adjacent said major surface and extends around said active device region to form with said body portion an auxiliary p-n junction located within the spread of a depletion layer from the reverse-biased main p-n junction and thereby serving to increase the breakdown voltage of the main p-n junction. A passivating dielectric layer extends over an area of said major surface between the active device region and a surrounding region of the body portion which is located beyond said at least one annular region. An electrically resistive layer overlies the dielectric layer and is electrically connected to said active device region and to said surrounding region of the body portion via windows in the dielectric layer.
The annular region or regions forming the auxiliary p-n junctions increase the breakdown voltage of the main p-n junction by reducing the electric field in the depletion layer as it spreads to these regions. The annular regions act like a voltage divider for the reverse-bias voltage applied between the active device region and the body portion. The resistive layer electrically connected between the active device region and the body portion imposes a potential gradient over the surface of the underlying passivating dielectric layer to shield the passivating dielectric layer against the effects of external charges.
In most of the examples given in U.S. Pat. No. 4009483, the resistive layer is also connected to the annular regions at further windows in the dielectric layer so that the potential of the annular regions may be established by a voltage division effect caused by the leakage current flowing through the resistive layer. However, experiments performed in connection with the present invention have shown that connecting the resistive layer to the annular regions may often result in a reduction of the breakdown voltage, for example from 1000 volts to 600 volts. The precise mechanism or mechanisms responsible for this reduction are not fully known but appear to relate to the flow of leakage current between the annular region and the part of the resistive layer connected thereto.
Provided the resistance layer is connected to the body portion at a considerable distance beyond the (outermost) annular region, it appears that the potential distribution along the resistive layer can reinforce the reverse-biasing of the annular region(s) in the depletion layer. This tends to raise the potential of the annular region(s) so that only a small reverse leakage current is taken from the resistive layer which then seems to function as a voltage divider. However the need to provide this considerable distance beyond the (outermost) annular region may result in an undesirable increase in the semiconductor body surface area needed for the device.
The connection of the resistive layer to the body portion may be brought close to the (outermost) annular region, for example as illustrated in FIG. 2 of U.S. Pat. No. 4009483. In this situation, however, it appears from analyses performed by the present inventor that the potential distribution along the resistive layer will try to decrease the potential at which the annular region(s) would otherwise float in the depletion layer so that larger (even forward) currents may be taken from the resistive layer by the annular region(s). The current flow along the resistive layer may then become non-uniform so that undesirably different potential gradients may occur over the various lengths of the resistive layers resulting in less homogeneous electric fields across the dielectric layer and premature breakdown.
It is suggested at column 5, lines 31 to 46 of U.S. Pat. No. 4009483 that, if high leakage currents are anticipated for a particular manufactured device, it may be desirable to eliminate the contact between the resistive layer and the annular region(s). Such modified device structures have been fabricated in which the resistive layer is insulated from the annular region(s) by the dielectric layer extending continuously between the window where connection is made to the active device region and the window where connection is made to the outer surrounding region of the body portion. In general these devices were found to have breakdown voltages which were higher, although generally still slightly less than what was expected given the resistivity of the body portion, the number of annular regions and the dimensions and other parameter values chosen for the design of the device. As its intermediate connections have been eliminated, the potential along the resistive layer no longer depends on its conductivity and a constant current flows along the length of the resistive layer. However it is found that during the first few seconds after application of the reverse-voltage the breakdown voltage is slightly lower than the final value achieved, e.g. increasing from an initial 1000 volts to about 1200 volts with particular device structures designed for operation up to such voltage levels. The present inventor believes that this effect may result from a significant degree of mismatch between the uniform potential variation along the resistive layer and the non-uniform potential variation along the semiconductor body surface including the annular region(s). This potential mismatch is thought to produce high vertical fields through the intermediate dielectric layer resulting in local charging of the dielectric layer. The effect is found to be particularly problematic with wide closely-spaced annular regions such as may be used, for example, adjacent the active device region in accordance with the teaching in co-pending European patent application No. 83.201681.0.